Method of lithium sulfate and sodium (potassium) carbonate directly producing lithium carbonate and reducing sulfate radical content

ABSTRACT

Traditional methods for producing lithium carbonate involves thermal precipitation of a lithium sulfate purification liquid and a sodium (potassium) carbonate purification liquid to produce crude lithium carbonate to the production of a refined lithium carbonate wet product. The employment of “reverse feeding, non-circulating mother liquor”, “pre-precipitation supplementary impurity removal” and “high-efficiency desorption” can reduce industrial grade lithium carbonate sulfate radicals to 0.03%, increase the main content to 2.5N, reduce battery grade sulfate radicals to 0.008%, and stably increase the main content to 3N, or even reach the limit of 3.5N-4N. The high-efficiency desorption involves thermal precipitation with small temperature increases and thermal stirring washing, medium-high temperature strong desorption, and hydrocyclone separation. Impurities such as sulfate radicals that are chemically adsorbed and encapsulated in the peritectic core of lithium carbonate particles can be released into deionized water, which are then effectively carried away by a hydrocyclone separation liquid phase.

I. TECHNICAL FIELD

The invention relates to a method for producing a lithium salt.Particularly, the invention relates to a new method for substantiallyreducing the content of sulfate radicals in lithium ores such asspodumene, lepidolite, zinnwaldite, primary lithium carbonate fromcarbonate salt lake lithium ore, amblygonite, petalite, and carbonatesedimentary rock lithium ore; intermediate lithium sulfate prepared by asulfuric acid method, a sulfate method or a sulphide method; and variousgrades of lithium carbonate directly produced; and more particularly,the invention relates to a new method for substantially reducing thecontent of sulfate radicals in lithium carbonate obtained by the thermalprecipitation reaction of a lithium sulfate solution with a sodium(potassium) carbonate solution. In order to simplify the contents of thespecification, a spodumene-sulfuric acid method is exemplified only fordetailed illustration.

The contents of the invention, especially the similar fields to whichthe third technique of “high-efficiency desorption” can naturallyextend, are detailed in paragraph [0081].

II. BACKGROUND

At the beginning of the last century, the industrial-grade lithiumcarbonate produced in Europe generally contained 0.70-0.80 wt. %impurity sulfate radicals (unless otherwise stated, the percentage orproportion described herein refers to a mass percentage), equivalent to1.035%-1.183% sodium sulfate, with an arithmetic mean of 1.109%, whichappears to be much higher than that of many other water-insoluble andslightly water-soluble carbonate products.

In the 1940s and 1950s, the former Lithium of America corporationinvented a process for producing lithium carbonate by thespodumene-sulfuric acid method, and built a factory with an annualoutput of 9000 tons in Bismarck, N.C., in whose industrial-grade lithiumcarbonate standard, the impurity sulfate radicals had been substantiallyreduced as compared to that in the early European products, with acontent of 0.35% for grade I products and 0.50% for grade II products,which, however, is still high for the medium-to-high-gradelithium-containing glass and other industries.

When the production of medium-to-high-grade lithium-containing glasssuch as glass ceramics requires lithium carbonate with a content ofsulfate radicals as low as 0.20% (i.e., Dow Corning glass standard), theTruste method is used for the purification. Such method comprises thesteps of pressing carbon dioxide into lithium carbonate aqueous slurryprepared by 20 times of deionized water to acidify the lithium carbonateinto a solution of 5% lithium bicarbonate in water, diluting impuritysodium sulfate in a large amount of water, and then heating the solutionto decompose the lithium bicarbonate, removing carbon dioxide, andre-precipitating lithium carbonate in a low-concentration sulfateradicals environment to achieve the aim of reducing the content ofsulfate radicals to 0.20%. However, the purification process has a longflow, and requires huge equipment investment, resulting in a greatlyincreased cost.

The chemical components of the industrial-grade lithium carbonatespecified by the current Chinese National Standard GB/T11075-2013 areshown in Table 1 below:

TABLE 1 Chemical component (mass fraction)/% Main Content of impurities,not more than content of Hydrochloric Product Li₂CO₃, not acid insolublebrand less than Na Fe Ca SO₄ ^(2—) Cl^(—) matter Mg Li₂CO₃-0 99.2 0.080.0020 0.025 0.20 0.010 0.005 0.015 Li₂CO₃-1 99.0 0.15 0.0035 0.040 0.350.020 0.015 — Li₂CO₃-2 98.5 0.20 0.0070 0.070 0.50 0.030 0.050 —The water content of the product shall meet the requirements as shown inTable 2 below:

TABLE 2 Product brand Li₂CO₃-0 Li₂CO₃-1 Li₂CO₃-2 Water content, 0.3%0.3% 0.5% not more than

Current Nonferrous Metals Industry Standard of China for thebattery-grade lithium carbonate

The chemical components specified in YS/T582-2013 are shown in Table 3below:

TABLE 3 Content of Content of impurities, not more than Li₂CO₃ Na Mg CaK Fe Zn Cu Pb Si Al Mn Ni SO₄ ^(2—) Cl^(—) ≥99.5 0.025 0.008 0.005 0.0010.001 0.0003 0.0003 0.0003 0.003 0.001 0.0003 0.001 0.08 0.003

As can be seen from the chemical component tables of lithium carbonatespecified in GB/T11075-2013 and YS/T582-2013, the indexes of impuritysulfate radicals in Li₂CO₃-0, Li₂CO₃-1 and Li₂CO₃-2 lithium carbonateare 1-2 orders of magnitude higher than the indexes of other impuritiesFe, Ca, Mg and Cl; in the battery-grade lithium carbonate, the indexesof impurity sulfate radicals are 2-3 orders of magnitude higher thanthat of Mg, Ca, Fe, Zn, Cu, Pb, Si, Al, Mn, Ni and Cl, showing an evengreater difference. Obviously, the reason for this is that the technicaldifficulty in reducing the content of sulfate radicals is greater thanthat in reducing the content of other impurities. It should be knownthat it took about 40 years to reduce the content of impurity sulfateradicals from 0.7%-0.8% in industrial-grade lithium carbonate producedby the earliest sulfuric acid (sulfate) method in Europe to 0.50%-0.35%in industrial-grade lithium carbonate produced by the sulfuric acidmethod of the former Lithium of America corporation. So far, it hastaken more than 100 years to reduce the content of impurity sulfateradicals to 0.20% in the common industrial-grade lithium carbonate and0.08% in the battery-grade (also essentially an industrial-grade)lithium carbonate. Therefore, it can be imagined how the process forreducing the content of impurity sulfate radicals is difficult.

Chinese Patent Application No. CN107915240A (disclosed on Apr. 17, 2018)discloses a process for producing battery-grade lithium carbonate by asulfuric acid method, wherein the contents of the indexes of impuritiessulfate radicals and sodium are 0.08% and 0.025%, respectively, whichare still high. If the contents can be reduced by one more order ofmagnitude, it will be very beneficial to improving the quality oflithium batteries.

III. SUMMARY

The present application only takes the spodumene-sulfuric acid method asan example for illustration. It should be understood that this shouldnot be construed as limiting the protection scope of the invention, andany techniques that can be implemented based on the contents of theinvention are included in the intended protection scope of the presentapplication.

The technical problems to be solved by the invention are as follows: (1)on the basis of the current techniques for directly producing thebattery-grade lithium carbonate by the thermal precipitation processwith a purified lithium sulfate solution and a purified sodium(potassium) carbonate solution and the product standard YS/T582-2013,innovating some processes to substantially reduce the content ofimpurity sulfate radicals to 0.010%-0.008% and slightly reduce thecontent of other impurities, thereby stably increasing the main contentof the battery-grade lithium carbonate to 3N grade, and under optimizedconditions, to 3.5N grade, and nearly or even finally to 4N grade (canbe referred to as “quasi-high purity grade”) for some products. Theinventor of the present application believes that the limit of the maincontent of the lithium carbonate produced directly by the thermalprecipitation process with lithium sulfate solution and sodium(potassium) carbonate solution may be 4N.

(2) On the basis of the current techniques for producing theindustrial-grade lithium carbonate by the thermal precipitation processwith a purified lithium sulfate solution and a purified sodium(potassium) carbonate solution and the product standard GB/T11079-2013,innovating some processes to substantially reduce the content ofimpurity sulfate radicals to 0.03% for the “new zero-grade” lithiumcarbonate and the content of sodium and other impurities, therebyincreasing the main content to 99.50%; and to substantially reduce thecontent of impurity sulfate radicals to 0.10% for the “new first-grade”lithium carbonate, thereby increasing the main content to 99.35%. Otherlow-grade industrial lithium carbonates are not considered, because oncethe three techniques of the invention are fully implemented, no sulfateradical with a content higher than 0.10% is present in the producedindustrial lithium carbonate.

The technical schemes for solving the above technical problems are asfollows: according to different requirements on product quality and onthe basis of the current techniques for producing the industrial-gradeand battery-grade lithium carbonates, applying the correspondingcombinations of the following three techniques of the invention toachieve the aims described in paragraphs [0010]-[0011]: 1, “reversefeeding without mother liquor circulation”; 2, “supplementary impurityremoval by pre-precipitation”; and 3, “high-efficiency desorption”.

The technical schemes are detailed in sequence in following paragraphs[0013]-[0073].

To achieve the aims described in paragraphs [0010]-[0011], the first andthird techniques of the invention are essential, which are appliedstarting from the procedure of obtaining coarse lithium carbonate by athermal precipitation reaction of a completely purified lithium sulfatesolution with a completely purified sodium (potassium) carbonatesolution, and ending with the procedure of obtaining various grades ofrefined wet lithium carbonate; the second technique of the invention isoptional, which is used prior to the thermal precipitation reaction,primarily for the production of industrial-grade lithium carbonate, andoptionally for the production of battery-grade lithium carbonate ifnecessary.

The combinations of the three techniques of the invention are asfollows:

1. When the current industrial zero-grade lithium carbonate with 0.20%sulfate radicals needs to be produced, the technique for removingimpurities such as silicon, iron, aluminum, magnesium, calcium and heavymetals before the current thermal precipitation process remainsbasically unchanged, and all detection methods remain unchanged; byapplying the combination of the first and second techniques of theinvention, various solid lithium ores such as spodumene, lepidolite,primary lithium carbonate from carbonate salt lake lithium ore (someproduced in the Grag Yer Tshwa Kha, Lung Mu Mtsho, Jieze Chaka Salt Lakeof Tibet, and some stored in the Atacama salt lake of South America),zinnwaldite, phospholithite, petalite, and (future) carbonatesedimentary rock lithium ore in Yunnan province, China, (future) hardrock lithium ore in Afghanistan, and lithium-containing waste recoveredfrom lithium batteries can be purified into a completely purifiedlithium sulfate solution, which is then subjected to a thermalprecipitation reaction with a completely purified sodium (potassium)carbonate solution to directly produce the lithium carbonate needed.2. When the industrial “new first-grade” lithium carbonate with 0.10%sulfate radicals needs to be produced, the combination of the firsttechnique, (or optional) second technique, and the front procedure“thermal precipitation with small temperature increases and thermalstirring washing” of the third technique of the invention is applied(see paragraphs [0024], [0027]-[0029], [0041], and [0042] for detaileddescription).3. When the industrial “new zero-grade” lithium carbonate with 0.03%sulfate radicals needs to be produced, the combination of the first andthird techniques in combination with the second technique if necessaryof the invention is applied on the basis of various impurity removaltechniques for the current industrial-grade lithium carbonate.4. When the “new battery-grade” lithium carbonate with 0.01%-0.008%sulfate radicals needs to be produced, the combination of the first andthird techniques (and optional the second technique for supplementationif necessary) of the invention is applied on the basis of variousimpurity removal techniques for the current battery-grade lithiumcarbonate (the selected operation parameters such as the deionized wateramount, the temperature-pressure parameters, and the thermal agingduration in the third technique are more stringent than those in theproduction of industrial “new zero-grade” lithium carbonate) to obtainthe lithium carbonate needed.

From this paragraph to paragraph [0072], the three techniques of theinvention each are further detailed by taking the spodumene-sulfuricacid method for directly producing lithium carbonate as an example:

The “reverse feeding without mother liquor circulation” was invented andnamed by the inventor of the present application when he was in chargeof the spodumene-sulfuric acid method for directly producing lithiumcarbonate with 0.22%-0.15% sulfate radicals special for a certainenterprise in a certain chemical plant in 1978-1980 in Chengdu, China(see the annex of Chinese Patent Application No. 201810900977.7). Itrefers to:

1. Starting from the thermal precipitation procedure with the completelypurified lithium sulfate solution and the completely purified sodiumcarbonate solution, the classic operation of the spodumene-sulfuric acidmethod invented by the former Lithium of America corporation, namely,adding a “saturated solution of Na₂CO₃ as a precipitant” into a “20%Li₂SO₄ solution” (which is referred to as “forward feeding” in thepresent application) as shown in FIG. 1, is reversed, and the completelypurified lithium sulfate solution is added into the purified sodiumcarbonate solution being vigorously stirred at 90-95° C. throughproperly dispersed feeding points, so as to precipitate coarse lithiumcarbonate particles with fewer sulfate radicals that are chemicallyadsorbed and deeply coated.

When the industrial “new first-grade”, “new zero-grade” and “newbattery-grade” lithium carbonates needs to be produced, the temperatureof the thermal precipitation and stirring washing operation is increasedto 104.8-120.2° C. (corresponding to a saturated steam pressure of0.13-0.20 MPa), which is one of the innovative measures more beneficialto reducing the content of sulfate radicals in lithium carbonate. Seeparagraphs [0014], [0027-0029], [0032] and [0041] for detaileddescription.

2. Moreover, instead of following the classic operation of the formerLithium of America corporation, namely, cooling the primary hot motherliquor of sodium sulfate, which has been centrifuged while hot to obtainthe coarse lithium carbonate, to 0° C. to −15° C. to crystallizemirabilite, and sending the secondary cold mother liquor back to theacidification material leaching process to recover lithium; anotherprocess route is developed: the secondary cold mother liquor containinglithium (converted into lithium carbonate) with a concentration up to10-15 g/L is heated and concentrated until a sodium sulfatecrystallization film is exactly formed on the liquid surface (a smallexcess of sodium carbonate is retained in the mother liquor, and coarselithium carbonate is gradually precipitated in the concentrationprocess), filtered while hot to obtain the coarse lithium carbonate, andsent back to the acidification material leaching process; the tertiaryhot mother liquor from which the coarse lithium carbonate is filteredout is combined with the new primary hot mother liquor from which thecoarse lithium carbonate is precipitated, and cooled to precipitatemirabilite, as such, the operations of “cooling for precipitatingmirabilite-heating for precipitating the coarse lithium carbonate” arealternately performed; alternatively, the secondary cold mother liquor(even the primary hot mother liquor) is directly concentrated in vacuumto recover anhydrous sodium sulfate after lithium is recovered therefromby precipitating lithium phosphate, lithium fluoride and organic acidlithium (such as lithium stearate).

The “without mother liquor circulation” means that a large amount oflithium-containing sodium sulfate mother liquor is not sent back to theleaching process, as such, the content of harmful sodium sulfate in theleached lithium sulfate solution system is minimized, and theconcentration of sulfate radicals in the thermal precipitation reactionsolution of the lithium carbonate is further reduced. Then, thebeneficial technical effect of the “without mother liquor circulation”and the beneficial technical effect of reducing the chemical adsorptionand deep coating of the sulfate (hereinafter referred to as “peritecticcrystal”) by the “reverse feeding” are superposed to each other; andsince the concentration of sodium sulfate in the purified lithiumsulfate solution is reduced a lot and the salt effect is reduced, thefirst pass yield of the coarse lithium carbonate from the thermalprecipitation is increased.

The beneficial technical effects generated by the “reverse feedingwithout mother liquor circulation” are realized at very low equipmentand investment costs, and heretofore, there has been no economical andeffective method in the art that can conveniently reduce the content ofsulfate radicals in lithium carbonate produced directly by thespodumene-sulfuric acid method to 0.35% or less. Moreover, thistechnique is still another inventive improvement created later tofurther substantially reduce the content of sulfate radicals, namely, anindispensable supporting and foundation technique for the thirdtechnique “high-efficiency desorption” of the invention.

The “supplementary impurity removal by pre-precipitation” comprises thefollowing three applications:

1) If it is not found until the beginning of the thermal precipitationprocedure that there is a mistake in the leaching operation or theoperation of removing impurities such as aluminum, iron, magnesium,calcium, and heavy metals by successive precipitation in the earlystage, such that colloidal particles formed of certain impurities arenot fully coagulated and thus incompletely precipitated, the impuritiespass through the filter since the filter cloth is damaged and improperlyplaced, or the circulation of filtrate is interrupted before theresidues are successfully bridged due to the improper filtrationoperation, and the indexes of the impurities of the purified lithiumsulfate solution are detected to be unqualified, then a small amount ofthe purified sodium carbonate solution can be slowly added into thepurified lithium sulfate solution under stirring in the “forwardfeeding” mode prior to the thermal precipitation operation, the additionis discontinued once white fine precipitates appear as closely detectedby naked eyes and a turbidity meter, and the solution is continued to bestirred for a few minutes, and then the purified lithium sulfatesolution which is discharged from a sampling port and carefully filteredis detected; if the indexes of these impurities are qualified, theaddition is stopped (if the indexes of these impurities are stillunqualified, a small amount of purified sodium carbonate solution issupplemented until the indexes are qualified), and the solution iscontinued to be stirred for more than 15 minutes to ensure thataluminum, iron, magnesium, calcium, certain heavy metal hydroxides andcalcium carbonate exceeding the standard are fully coagulated andco-precipitated, and then filtered in vacuum. Since the filtrate isturbid in the initial stage, it is necessary to pump out the filtratefor circulating filtration until a filter cake is successfully bridged,and when the filtrate is observed to be completely clear, thecirculating filtration is stopped. If it is observed that the obtainedfilter cake has a fine and smooth texture (mainly magnesium hydroxideand aluminum hydroxide), and has a small amount of coarse particles(lithium carbonate), the above impurities are well purified, and thepurified lithium sulfate solution which is successfully filtered isconfirmed to be a completely purified solution after another detection.Compared with the procedure of sending the unqualified purified lithiumsulfate solution back to the leaching process to remove the impuritiessequentially, this simple error correction and rescue method leads to amore significant beneficial technique and cost effect, especially forsmall manufacturers with poor conditions such as equipment andmanagement.

After completing the “supplementary impurity removal bypre-precipitation”, the production of the current industrial zero-grade,“new zero-grade” and “new first-grade” lithium carbonates as well as thebattery-grade lithium carbonate must comprise the thermal precipitationoperation in the “reverse feeding without mother liquor circulation”mode.

2) By applying the combination of the “reverse feeding without motherliquor circulation”, the “supplementary impurity removal bypre-precipitation” and the “thermal precipitation with small temperatureincreases and thermal stirring washing” in the “high-efficiencydesorption” (see paragraph [0014]), as well as the current impurityremoval techniques before the thermal precipitation procedure, thecontent of sulfate radicals in the industrial-grade lithium carbonateproduced by the spodumene-sulfuric acid method can be readily reduced toa limit of 0.35% below, and can be reduced to 0.15% (AR standard) oreven 0.10% (the standard for special-grade lithium carbonate produced bythe reconversion of lithium hydroxide by the spodumene-lime method inthe Xinjiang Lithium Salt Plant in China; now considered to be veryclose to the current battery-grade standard), leading to the industrial“new first-grade” lithium carbonate.

3) In the previous production process, it is often observed thathydroxides of iron, aluminum and magnesium are flocculated andprecipitated in the completely clear purified lithium sulfate solutionduring the concentration, which is consistent with the phenomenonrecorded in the technical literature of the spodumene-lithium sulfatemethod for producing lithium carbonate disclosed by the former Lithiumof America corporation, indicating that it is necessary to perform fullflocculation, co-precipitation and multiple removal on the colloidalimpurities passing through the filter and remained in the purifiedlithium sulfate solution. Particularly, when producing the battery-gradelithium carbonate (including other high-purity lithium carbonatevarieties), if a process of circular leaching without lithium sulfateconcentration is adopted, the colloid impurities of the hydroxides ofaluminum, iron, magnesium and certain heavy metals may not be heated fora long time or the surface charges of colloid particles may not beeliminated and sufficiently condensed, and thus may pass through thefilter, as such, the “supplementary impurity removal bypre-precipitation” technique can be adopted to remove the impurities ina supplementary way before the thermal precipitation of the coarselithium carbonate.

The “high-efficiency desorption” comprises two parts of “strongdesorption” and “hydrocyclone separation”. It is a powerful newtechnique required for further reducing the contents of sulfate radicalsin the industrial-grade lithium carbonate and battery-grade lithiumcarbonate to the maximum extent from 0.20%-0.15%-0.10% and 0.08% to0.03%-0.02% and 0.01%-0.008%, respectively, and increasing the maincontents to 99.50% and 3.5N-4N, respectively.

The “strong desorption” further comprises two parts of “thermalprecipitation with small temperature increases and thermal stirringwashing” and “medium-high temperature strong desorption”.

The “thermal precipitation with small temperature increases and thermalstirring washing” is also an improvement based on the technicalprinciple described in paragraphs [0057]-[0072] to the first and thirdtechniques of the invention: throughout the process of removing sulfatesin peritectic crystal by an operation system of the thermalprecipitation and thermal stirring washing and the “strong desorption”,adsorption and desorption are in a dynamic equilibrium, and theadsorption is exothermic and the desorption is endothermic, when thetemperature is increased, the equilibrium shifts to the desorptiondirection (the Le Chatelier's principle), and thus the desorption isfacilitated, that is to say, the desorption effect is positivelycorrelated with the temperature, and is a continuous change process. Theobvious effect of increasing the desorption of sulfate radicals can beapproximately predicted even if the temperatures of the thermalprecipitation and thermal stirring washing are increased by 10° C. byreferring to the data of the small experiment 1) in paragraph [0032].Moreover, the curvature of the lithium carbonate solubility-temperaturecurve is negative, and the first pass yield can be improved as thetemperature is increased.

The allowable pressure of the current common jacketed reactor is mostly0.6 MPa for the jacket, and 0.2 MPa for the reactor. Accordingly, thecontent of sulfate radicals in the coarse lithium carbonate 1 can beeffectively reduced by the thermal precipitation and thermal stirringwashing operation with only part of operation modes and operationparameters changed under the premise of not changing the current mainequipment, which lays a better technical foundation for the subsequent“medium-high temperature strong desorption” operation. This effect canbe predicted by referring to the results of the small experiment 1) inparagraph [0032]. In addition, this process improvement also reduces theamount of expensive deionized water used due to the reduced content ofsulfate radicals in the coarse lithium carbonate 2 in the reactor. Forthis reason, it is recommended to operate in the range of 104.8° C.(0.13 MPa)−115.2° C. (0.18 MPa)−120.2° C. (0.20 MPa), but this is notintended to limit other equipment to operate only in thistemperature-pressure range.

However, there is no need to increase the temperature excessively duringthe “thermal precipitation with small temperature increases and thermalstirring washing” operation. On one hand, the technical effect ofreducing the content of sulfate radicals is limited in the environmentwith high concentration of sulfate radicals, on the other hand, there isno need for a jacketed reactor with a higher allowable pressure as areplacement accordingly.

The “medium-high temperature strong desorption” refers to such aninventive invention for directly producing the battery-grade orquasi-high purity-grade lithium carbonate with 0.008% sulfate radicals,or even more possibly producing 4N-grade lithium carbonate with near0.005%-0.003% (50-30 ppm) sulfate radicals that: in cases where thesulfate radicals in the peritectic core of the coarse lithium carbonateparticles are difficult to reduce by the conventional thermal stirringwashing-centrifuging method, focuses on greatly increasing thetemperature to intensify the thermal motion of various molecules, ionsand radicals in a coarse lithium carbonate-2-deionized water slurrysystem, loosen coordinate bonds between the sulfates and lithium ions inthe peritectic core of the lithium carbonate particles to substantiallyseparate the sulfate radicals from the lithium carbonate particles, andrelease and dissolve the sulfate radicals in a large amount of deionizedwater; and to intensify the thermal motion of a considerable part of theother water soluble, slightly water soluble and water insolubleimpurities to release, dissolve and suspend the sulfate radicals in alarger amount of deionized water. The technical principle of adsorptionand desorption on which it is based is detailed in paragraphs[0057]-[0072]; and the operation instructions are detailed in paragraphs[0042]-[0043].

As proved by the results of the following two easily reproducible smallexperiments of the “strong desorption” by gradually increasing pressureand temperature conducted by the inventor of the present application inthe early years, the invention does have a strong desorption effect onthe sulfate radicals in the peritectic crystal of lithium carbonateparticles.

1) A proper amount of tap water was added into a household pressurecooker; the industrial-grade coarse lithium carbonate produced by the“reverse feeding without mother liquor circulation” process was firstlywashed with 3 times of hot distilled water once to reduce the content ofsulfate radicals to 0.40% or less, and added into a stainless steel cupplaced in the cooker, and then 3 times of distilled water was added; thestainless steel cup was covered to prevent the pollution by the tapwater outside the cup, and externally heated to a pressure of 0.12 MPa(the highest pressure of the household pressure cooker, corresponding tothe saturated steam pressure of about 105° C., and characterized in thata nozzle sprays steam to make a sudden sound), and static strongdesorption and thermal aging were performed for 1 hour. After naturallycooling and pressure reduction, the liquid phase was removed bydecantation, and 1 time of distilled water was added for the thermalstirring washing once; the refined lithium carbonate was measured forthe content of sulfate radicals by the chemical method, and the contentof sulfate radicals was substantially reduced to 0.15% or less andexceeded the expected value (0.20%). This experiment correspondssubstantially to the “thermal precipitation with small temperatureincreases and thermal stirring washing”.

2) The experiment was performed using a simple stainless steelhot-pressing cylinder at increased pressure-temperature: theindustrial-grade coarse lithium carbonate was firstly washed with 3times of hot distilled water once to reduce the content of sulfateradicals to 0.35%, and the strong desorption and thermal aging wereperformed with 6 times of distilled water under the pressure of 0.4-0.6MPa (corresponding to a saturated steam pressure of about 146-160° C.)for 1 hour, and then 1 time of distilled water was added for stirringwashing once; the fine industrial-grade lithium carbonate was measuredfor the content of sulfate radicals by the chemical method, and thecontent of sulfate radicals was reduced to 0.04%-0.035%, showing aprecipitous drop, and was lower than the standard of 0.08% for thecurrent battery-grade lithium carbonate, suggesting that the beneficialtechnical effects are significant. This experiment correspondssubstantially to the “medium-high temperature strong desorption”.

According to the results of the above small experiments, it can bepredicted that after the addition of the “strong desorption” and“hydrocyclone separation” techniques for removing impurities, thecontent of sulfate radicals can be further substantially reduced to0.03%-0.02% for the industrial-grade lithium carbonate for manufacturersthat produce lithium carbonate by the spodumene-sulfuric acid methodwith average current processes, equipment and management; and thecontent of sulfate radicals can be completely reduced to 0.008% for thebattery-grade lithium carbonate for manufacturers with advanced currentprocesses, equipment and management. This is mainly because the “strongdesorption” technical principles on which they are based are completelyidentical.

The “hydrocyclone separation” refers to a simple, low-investment, andeasy-to-operate solid-liquid separation technique that can efficientlyseparate out impurities such as sulfate radicals, which are separatedfrom lithium carbonate particles in the “strong desorption” operationand dissolved and suspended in a large amount of deionized water. It issuperior to various solid-liquid separation techniques using filtrationdevices, in that by this hydrocyclone separation technique, the majorityof the particulate water-insoluble impurities that are separated andsuspended in a relatively large amount of deionized water are broughtout directly with the rotating flowing liquid phase; while in thesolid-liquid phase separation by techniques using filtration devices,most of the particulate water-insoluble impurities will be trapped inthe solid-phase lithium carbonate, failing to live up to the beneficialtechnical effects of the “strong desorption”.

The schemes of the invention for solving the technical problems isfurther, systematically and completely described in the following:

(1) For the direct production of the current industrial zero-grade, “newzero-grade” and “new first-grade” lithium carbonates and battery-gradelithium carbonate by adopting a thermal precipitation method with alithium sulfate solution and sodium carbonate solution (after the“high-efficiency desorption” technique is applied, industrialsecond-grade and first-grade lithium carbonates have no meaning andvalue), the current techniques for removing impurities such as silicon,aluminum, iron, magnesium, calcium, heavy metals and magnetic metalsbefore the thermal precipitation process of a manufacturer remainbasically unchanged; the “supplementary impurity removal bypre-precipitation” technique of the invention can be used forsupplement; the current techniques for drying, crushing, metering andpackaging the refined wet lithium carbonate also remain unchanged; andall detection methods remain unchanged.

(2) The sodium carbonate in thermal precipitation process is 5% excess,that is, equivalent ratio of sodium carbonate:lithium sulfate=1.05:1.00,is relatively moderate. This is to reduce the adsorption capacity of thelithium carbonate particles to sulfate radicals during the thermalprecipitation reaction and to increase the first pass yield of thelithium carbonate. When used in the “cooling for precipitatingmirabilite-heating for precipitating coarse lithium carbonate” operationof the “without mother liquor circulation” process, the aboveformulation enables more coarse lithium carbonate to precipitateautomatically and the non-amphoteric metal element impurities in themother liquor system to be brought out as compared to the equivalentfeeding.

(3) The completely purified lithium sulfate and sodium carbonatesolutions must be subjected to the thermal precipitation and thermalstirring washing and the subsequent related operations in the “reversefeeding without mother liquor circulation” mode.

(4) The operations of the thermal precipitation process aresubstantially modified, that is, instead of pursuing the obtainment ofcoarse lithium carbonate particles with large particle size, we will tryto obtain those with small particle size, and move the thermal agingduration backward to the “medium-high temperature strong desorption”process to be completed together. This is to lay a foundation for the“medium-high temperature strong desorption” process to effectivelyrelease impurities such as sulfate radicals which are formed and deeplycoated in the initial stage of the thermal precipitation reaction, andthe technical principle on which the improvement is based is detailed inparagraphs [0069]-[0072].

(5) The “thermal precipitation with small temperature increases andthermal stirring washing” operation is performed as follows: thecompletely purified sodium carbonate solution is added to the reactorand heated, a manhole on the reactor is covered, and the reactor issealed after air in the reactor is completely driven out; when thereactor is heated to a selected temperature, the stirrer is started andis kept working effectively, the completely purified lithium sulfatesolution is sprayed into the reaction in a mist form under the pressureof 0.1-0.3 MPa through pressurized sprinklers arranged at multiplepoints (for producing lithium carbonate with 0.008% sulfate radicals,the lithium carbonate with small particle size needs to be obtained inthe procedure firstly, and is sprayed at a higher speed under thepressure of 0.3 MPa). Once the feeding is completed, the reactor isde-pressurized (a pipeline should be connected to recover steam heat),after which the reaction solution is immediately discharged, andcentrifuged and rinsed to obtain coarse lithium carbonate-1.

(6) The coarse lithium carbonate-1 obtained by the thermal precipitationis immediately transferred while hot into a reactor which has been addedwith deionized water with selected times, such as 3-4-5 times for theindustrial-grade lithium carbonate and 5-6 times for the battery-gradelithium carbonate, and heated to 90-95° C. with a stirrer started; amanhole is covered, with the heating continued, the reactor is sealedafter air in the reactor is completely driven out, subjected to thethermal stirring washing for 15 minutes after being heated to the sametemperature as that of the thermal precipitation reaction in paragraph[0041], and de-pressurized (the pipeline is preferably connected torecover steam heat), after which the reaction solution is discharged,and centrifuged and rinsed to obtain industrial-grade coarse lithiumcarbonate-2 and battery-grade coarse lithium carbonate-2 with thecontent of sulfate radicals controlled to be 0.30%-0.20% and0.15%-0.10%, respectively, for later use.

(7) The deionized water with selected mass multiples of the coarselithium carbonate-2 is pumped into a medium-high temperature strongdesorption reactor, and stirred at low speed, and the coarse lithiumcarbonate-2 is added; the mixture is heated to a selected temperaturesuch as 159-170° C. (corresponding to the saturated steam pressure of0.6-0.8 MPa), and subjected to the strong desorption and the thermalaging for 1 hour or more under the conditions of the low-speed stirringand the low-speed motion state of a slurry solid phase, so thatwater-soluble impurities mainly comprising sodium sulfate, and slightlywater-soluble and water-insoluble impurities in the peritectic core ofthe lithium carbonate particle are released in the deionized water, andlithium carbonate crystals with small particle size are recrystallizeinto large crystals with the content of sulfate radicals of 0.03%-0.02%(for the industrial “new zero-grade” lithium carbonate) or 0.010%-0.008%(for the “new battery-grade” lithium carbonate) under the condition thatthe concentration of sulfate radicals is far lower than the thermalprecipitation reaction concentration.

(8) After the content of sulfate radicals is detected to be qualified,the medium-high temperature strong desorption reactor is de-pressurized(a pipeline should be connected to recover steam heat), and when thepressure is reduced to 0.05-0.06 MPa, the stirring speed is increased tomaintain a strong stirring state of the slurry, the slurry is pumpedinto a hydrocyclone separator with the speed controlled to continuouslyseparate the liquid phase from the solid phase; the separated liquidphase with the released (slightly) water soluble and water insolubleparticulate impurities is sent back to the leaching process to recoverlithium, and part of the liquid phase can be used to clean the filtercloth and equipment; only part of the liquid phase which is sufficientlycoagulated and subjected to precise filtration is allowed to be involvedin the thermal stirring washing of the coarse lithium carbonate-1 toproduce industrial-grade products, and is prohibited in the subsequentprocesses. The solid phase only needs to be centrifuged and rinsed (ifnecessary, the solid phase is subjected to the thermal stirring washingonce for the battery-grade lithium carbonate) to obtain the wet fineindustrial-grade lithium carbonate with the content of sulfate radicalsreduced to 0.03%-0.02% and the wet fine battery-grade lithium carbonatewith the content of sulfate radicals reduced to 0.008% or less.

(9) In cases where a pipeline desorber is adopted to automatically andcontinuously perform the “medium-high temperature strong desorption”operation, the pressure of the slurry is reduced to 0.05-0.06 MPathrough a de-pressurized storage tank with a stirrer and a cooling waterjacket, and the slurry is pumped into the hydrocyclone separator for theseparation operation with the speed controlled.

One of the methods for determining the end point of the “medium-hightemperature strong desorption” operation is as follows: sampling isperformed through a continuous sampling port specially arranged in thedesorber, the content of sulfate radicals in the liquid phase ismeasured by intermittent and repeated sampling detection or continuouson-line detection, the content of residual sulfate radicals of thesolid-phase lithium carbonate (dry basis) is calculated by a programcontrol computer according to the obtained content data, the amount ofthe coarse lithium carbonate-2 added and the content of sulfate radicalstherein, and the amount of the deionized water added, if it is confirmedto be qualified, the desorption end point can be determined.

Specific remarks regarding the technical parameters of the“high-efficiency desorption” are as follows: various operationparameters, such as water amount used for firstly washing the coarselithium carbonate-1, times of the first washing, control indexes of thecontent of sulfate radicals in the coarse lithium carbonate-2 for the“thermal precipitation with small temperature increases and thermalstirring washing” procedure; water amount used for desorption, controlindexes of the saturated steam pressure-temperature of a desorber,rotating speed for stirring or rotating speed of the desorber,desorption and thermal aging duration and the like for the “medium-hightemperature strong desorption” procedure; and discharge pressure for the“hydrocyclone separation” procedure, are set according to the grade oflithium carbonate to be produced, quality requirements of orders,component characteristics of starting materials, yield and cost control,safety production management and other factors; the parametersexemplified in the summary and the detailed description of the inventionare considered as a whole instead of a fixed range of rigid parameters,and they can be flexibly adjusted and controlled in the actual practice;therefore, these parameter sets are all included in the protection scopeof the invention.

For example (but not limited to), for producing the industrial-gradelithium carbonate, the amount of the deionized water can be distributedas appropriate according to a ratio of 2.5:5:0.5 or 1.5+1.5:5.5:0.5 forthe “thermal precipitation with small temperature increases and thermalstirring washing”, “medium-high temperature strong desorption”, andcentrifuging and rinsing, and the total amount of the deionized waterused in the three procedures being 8-9 times the amount of the finishedlithium carbonate is sufficient; and for producing the battery-gradelithium carbonate, the amount of deionized water can be distributedaccording to a ratio of 2.5:6:0.5 or 1.5+1.5:6.5:0.5, and the totalamount of the deionized water used in the three procedures being 9-10times the amount of the finished lithium carbonate is sufficient.

For another example (but not limited to), temperature-saturated steampressure in a desorber: although the desorption effect is positivelycorrelated with temperature-pressure, that is, the higher thetemperature-pressure, the easier and more numerous the separation of thesulfate radicals and other impurities, and the shorter the time requiredfor the separation; however, in this case, the equipment and maintenancecosts will be higher, and the management of the enterprise will be morecomplex. Taking factors such as product quality requirement, technicaleffect, investment amount, production capacity, cost, safety managementof pressure vessels, and current equipment conditions of variousmanufacturers into account comprehensively, it is recommended that forproducing the industrial “new zero-grade” and “new first-grade” lithiumcarbonates, a pressure of 0.5-0.6 MPa is adopted, although it is notlimited but unnecessary to exceed 0.6 MPa, only in the automatic andcontinuous operation using a pipeline desorber, a pressure more than 0.8MPa can be adopted, although it is not limited but unnecessary to exceed1.0 MPa; and it is recommended that for the battery-grade lithiumcarbonate, a pressure of 0.7-0.8-1.0 MPa is adopted, although it is notlimited but unnecessary to exceed 1.0 MPa, only in the automatic andcontinuous operation using a pipeline desorber, a pressure more than1.0-1.2 MPa can be adopted, it is not limited but unnecessary to exceedthe current low pressure/medium pressure vessel limit of 1.6 MPa.

Therefore, in paragraphs [0028], [0032]-[0033], [0041]-[0045], [0048],[0049], and [0053]-[0054], various technical parameters that aremoderately adjustable and related to the “high-efficiency desorption”technique should be included in the intended protection scope of thepresent application.

The liquid phases of water-soluble impurities mainly comprising sodiumsulfate, water slightly-soluble impurities, other colloidal impuritiesand other particulate water-insoluble impurities which are separatedfrom the coarse lithium carbonate-2 particles are separated by a“hydrocyclone separator”, which is a good selection of solid-liquidseparation equipment for large-scale industrial, automatic andcontinuous production in the “medium-high temperature strong desorption”procedure; if a liquid-solid phase separation with filter cloth (such ascentrifugal separation) is adopted, a lot of suspended particulatewater-insoluble impurities will be mixed into the solid phase, such thatthe otherwise excellent impurity removal effect of the “medium-hightemperature strong desorption” is substantially reduced.

The optional structure types of the desorber used in the “medium-hightemperature strong desorption” comprise: 1) a pressure reactor with alow-speed stirring and a heating and cooling jacket, standard equipment;2) a low-rotating-speed spherical or horizontal cylindrical desorber,standard equipment or self-design; or 3) a pipeline desorber mostlysuitable for manufacturers with large design capacity, self-design; 4)any types of desorber adopting dividing-wall heating and cooling insteadof direct steam heating to avoid polluting the slurry.

As the inner surface structure material of the desorber used in the“medium-high temperature strong desorption” procedure contacting thematerial, the titanium plate composite material is preferable, followedby the stainless steel plate made of 0Cr18Ni9Ti or 0Cr18Mo2Ti. However,in the production of the battery-grade lithium carbonate, if a stainlesssteel material is to be selected for the inner surface structurematerial of the desorber, it is necessary to make a small pressurereactor with pressure resistance of 1.6 MPa due to the strictrestriction of the content of magnetic metal chromium in the product ofless than or equal to 3 ppm, and a long-time (100 hours or more arerecommended) soaking test is performed firstly on lithium carbonateslurry under the condition of a saturated steam pressure of 0.8-1.0-1.2MPa in the reactor to detect the chromium leaching amount: as long asthe chromium content of the lithium carbonate after the soaking test isincreased by 1 ppm compared with that before the soaking, the materialof this batch cannot be selected, and another selection is needed. Inaddition, the soaking test is also required for lithium sulfate obtainedfrom fluorine (chlorine)-containing starting materials such asfluor-lepidolite, but this is to measure the corrosiveness of fluorine(chlorine) to the two materials, and if corrosion exists, a compositesteel plate lined with polytetrafluoroethylene (the allowabletemperature for long-term use is −180° C. to 260° C.) is selected as thestructure material.

For the scheme using the design of glass lining in the inner wall of thedesorber, it is necessary to perform a test with material firstly todetect the dissolution amount of elements such as boron, aluminum,silicon, lead, and antimony in the glass lining under the conditions ofalkaline lithium carbonate slurry, long time (100 hours or more arerecommended), high temperature (corresponding to the saturated streampressure of 0.8-1.0-1.2 MPa) and low-speed stirring: once the aboveelements and others soluble in alkali and limited to indexes ofimpurities of the battery-grade lithium carbonate dissolve out and leadto the material unqualified, the formula comprising the glass lining inthe inner wall should be rejected, and another selection is needed. Areactor with glass lining in the inner wall is not suitable for thelithium sulfate obtained from fluorine-containing starting materialssuch as fluor-lepidolite.

For the “hydrocyclone separation”, a hydrocyclone separator of standardequipment or self-design is used, the material of which is selected asthe same as the inner surface structure material of the desorber used inthe “medium-high temperature strong desorption” procedure contacting thematerial.

It is preferable to use the same material as the inner wall of the mainequipment for the pipe fittings of all procedures of the“high-efficiency desorption”.

The technical schemes of the invention have the following beneficialeffects: by using a relatively simple technical scheme, the content ofimpurity sulfate radicals and other impurities in the industrial-gradeand battery-grade lithium carbonates directly produced by the lithiumsulfate solution and the sodium (potassium) carbonate solution extractedfrom various lithium ores and sulfur-containing starting materialsreferred to in paragraph [0001] can be substantially reduced at lowproduction costs, and the main content of the two types of lithiumcarbonates is improved; the otherwise large and even huge quality, costand price difference of the industrial-grade and battery-grade lithiumcarbonates directly produced by a lithium ore-sulfuric acid method, asulfate method and a sulfur compound method and the high-purity gradelithium carbonates produced by various methods are substantiallyreduced, with the boundary blurred, so that the technical standard ofall grades of lithium carbonates to be modified in the future can besimplified. The beneficial effects are very beneficial to promoting therapid development of high-end lithium industries such as lithiumbatteries, and are very beneficial to enabling the long-term combinationof lithium ore salt with the high-quality and low-price salt lakelithium salt.

The technical principle on which the “reverse feeding without motherliquor circulation” and “high-efficiency desorption” are based isdescribed in paragraphs [0057]-[0072].

Paragraphs [0002]-[0008] have pointed out the long-existing problem thatthe impurity sulfate radical in lithium carbonates produced directly bythe lithium ore-sulfuric acid method and the sulfate method is toonumerous to remove. The essential reason is that lithium ions are likelyto form coordinate bonds with oxygen acid radicals containing silicon,carbon, and sulfur due to their structural characteristics, that is,sulfate radicals are likely to be chemically adsorbed to coarse lithiumcarbonate during the thermal precipitation to form coating (peritecticcrystal), so that the sulfate radicals are difficult to wash out.Particularly, the adsorbed sulfate radicals grow with the coarse lithiumcarbonate particles in the initial stage of the thermal precipitation,and will even be deeply coated, which is extremely difficult to removeaccording to the current thermal stirring washing method, causing thegreatest harm. Although the alkali and alkaline earth metal elements donot have as great polarizability as the transition elements, they canform a coordination complex with a coordinating atom as a central atom,particularly a lithium atom having the smallest radius among all metalelements, which is advantageous for forming a coordination complexhaving a slightly larger stability constant. A sulfate radical has twocoordinating oxygen atoms, so that the sulfate radical is conducive toforming a coordination complex with a slightly large stable constantwith a lithium ion in lithium carbonate, and the generated coordinationbond has slightly high energy and stronger chemical adsorption force(the principle is also suitable for carbonate and silicate radicals).

According to the Langmuir theory of solid surface adsorption in physicalchemistry, when coarse lithium carbonate particles are precipitated andwashed with a lithium sulfate solution and a sodium (potassium)carbonate solution at a relatively high temperature of 90-95° C., thephysical adsorption force based on van der Waals force is weak. Sincetwo coordinating oxygen atoms in the sulfate radical can be used ascoordination sites of the coordination complex, the probability ofgenerating a sulfate radical coordination complex with a slightly largestability constant is very high under the condition that theconcentration of the sulfate radical is high when the coarse lithiumcarbonate is precipitated. The adsorption of the sulfate radicals to thesurface of the coarse lithium carbonate particles is mainly chemicaladsorption, wherein the adsorbent is lithium ions, and the adsorbate issulfate radicals. The other characteristics of the chemical adsorptionare as follows: a. The selectivity is very high. During the thermalprecipitation, the lithium carbonate particles have strong adsorptionforce for both sulfate and carbonate radicals, and whether the sulfateradical or the carbonate radical is more likely to be adsorbed oradsorbed in larger amounts mainly depends on the concentration of theadsorbate, because the Freundlich adsorption formula shows that theadsorption capacity increases with the increase of the adsorbateconcentration. b. Only single-layer adsorption occurs. This is becausethe chemical adsorption is accomplished by forming a new chemical bondwith the adsorbate through the residual bond force of the molecules onthe surface layer of the solid molecules, and when the surface of thesolid molecules is saturated with the adsorbate, it will no longerabsorb the adsorbate with the same charge to form the second adsorptionlayer. c. The adsorption is exothermic and not easy to reverse, that is,the desorption is difficult to take place and is endothermic. Thechemical adsorption also promotes the coating of sulfate radicals in thecrystal growth process, because once the sulfate radicals are adsorbedon the lithium carbonate particles and are not easy to desorb, lithiumcarbonate molecules coordinated with the sulfate radicals are thenadsorbed outside to form the coating of the sulfate radicals, namelyperitectic crystals, so that the conventional washing method isdifficult to desorb and remove the sulfate radicals in the lithiumcarbonate particles, and the higher content of the sulfate radicals isinevitable.

In terms of the two factors of the adsorbate and the adsorbent, theformer has greater influence on the content of the impurity sulfateradicals in the lithium carbonate as proved by the production practice.

According to the above theoretical analyses, for the purpose ofsubstantially reducing the content of the impurity sulfate radicals, itis the most important to reduce the concentration of the adsorbatesulfate radicals in a thermal precipitation reaction system as much aspossible, and adopt the “slowing, heating and aging” operation in athermal stirring washing process to obtain refined lithium carbonateparticles with large particle size, so as to reduce the sulfate radicalsin the peritectic crystal; more importantly, it is necessary to find asimple, low-cost and powerful desorption technique to release thesulfate radical in the peritectic crystal which is difficult to removeby the current thermal stirring washing-centrifuging method.

Based on the above knowledge, the inventor of the present applicationproposed and led the implementation of the technical scheme of the“reverse feeding without mother liquor circulation” in 1978-1980. The“reverse feeding” is derived according to the principle that thechemical adsorption has the characteristics of selective adsorption,single-layer adsorption and difficult desorption: in the initial stageof feeding, the nascent lithium carbonate particles are in anenvironment with high concentration of carbonate radicals and lowconcentration of sulfate radicals, so that the surface is more likely toadsorb carbonate radicals rather than sulfate radicals, and onlyindividual sites adsorb the sulfate radicals (and a small amount ofsilicate radicals); due to the characteristic of the single-layeradsorption, after the surface of the lithium carbonate particles issaturated with the adsorbate carbonate radicals, it will no longeradsorb the electronegative sulfate and carbonate radicals. Since thecarbonate radicals adsorbed on the lithium carbonate are not easy toreversely desorb, but will quickly adsorb free electropositive lithiumions (followed by sodium ions), the carbonate radicals and lithium ionsare cross-adsorbed, lithium carbonate particles can quickly grow in anenvironment with lower concentration of sulfate radicals, and the amountof the adsorbed sulfate radicals is substantially reduced compared withthat in a “forward feeding” process.

The “without mother liquor circulation” substantially reduces theconcentration of sulfate radicals in a thermal precipitation reactionslurry system, and superpose the beneficial effect of reducing theadsorption of the sulfate radicals in lithium carbonate particles.

A layer of carbonate radicals adsorbed by the precipitated lithiumcarbonate particles partly adsorbs sodium ions to form sodium carbonate,which will not cause too much trouble: on one hand, these carbonateradicals can chemically adsorb lithium ions dissociated from thecontinuously added lithium sulfate to further generate lithium carbonatewith much lower solubility than sodium carbonate and more firm bondingby chemical reaction, so that lithium carbonate particles become larger,and sodium ions which are excluded and exchanged by the lithium ionsadded into a thermal precipitation system later can be absorbed by freesulfate radicals in a reaction solution and transferred into thereaction solution; on the other hand, the sodium carbonate and thelithium carbonate do not generate a complex salt, which has much highersolubility in hot water than lithium carbonate, and can be easily washedout when stirred and washed with hot water in a post-process. Of course,there will also be a small amount of sodium ions that adsorb sulfateradicals to form sodium sulfate, which is then deeply coated by lithiumcarbonate adsorbed thereon. The product lithium carbonate is sometimesfound to contain slightly less sodium than the equivalent amount ofsulfate radicals, indicating that there are trace amount of sulfates ofother metal elements, such as calcium, coated. These sulfate radicalsare more difficult to wash out.

The “reverse feeding” process, by means of the preferential complexingof high concentration of adsorbate carbonate radicals with lithium ionsin nascent lithium carbonate particles, prevents adsorbate sulfateradicals from complexing with a large number of adsorbent lithium ionsin the lithium carbonate particles and thus being coated, and thussuccessfully reduces the content of sulfate radicals in the product onan industrial production scale. After the “reverse feeding” (incombination with the “supplementary impurity removal bypre-precipitation”) is adopted, deionized water only needs to be addedto the coarse lithium carbonate in a ratio (mass ratio) of 1:2 to 1:3for the thermal stirring washing-centrifuging for 3 times to obtain aproduct with 0.15%-0.20% sulfate radicals.

The purpose of performing the “slowing, stirring, heating and aging”operation in the current thermal precipitation procedure of coarselithium carbonates is to obtain lithium carbonate particles with largeparticle size, so as to reduce the adsorption and the coating of sulfateradicals. The principles on which it is based include: 1, the Langmuirtheory, means that the smaller the surface of the adsorbent, namely thelarger the particle size, the lower the adsorption capacity; 2, theKelvin formula, means that aging enables small crystals to convertautomatically into large crystals (the system tends to be stable due tothe reduced free energy), and in the conversion process, under thestirring and heating conditions, part of the adsorbed and coated sulfateradicals and sodium ions can be released into the reaction solution;however, the sulfate radicals adsorbed by the nascent lithium carbonateparticles have been deeply coated in the early stage of the reaction,therefore, the amount of the sulfate radicals adsorbed and coated in thelithium carbonate particles is still very large in a dynamic reversiblestate of adsorption-desorption in the later stage of the reaction evenif the concentration of the sulfate radicals in the reaction solutionhas been very high, and thus a new technological breakthrough is stillneeded to solve this problem; 3, the Le Chatelier's principle, meansthat increasing the temperature is conducive to the desorption.

In the initial stage of the thermal precipitation of lithium carbonate,particularly under the conditions of fast feeding and weak stirring, thethermal precipitate is often very viscous for the following reasons: a.Four main ions in the lithium sulfate and sodium carbonate solutionsused in a thermal precipitation reaction have high concentrations andstrong reaction tendency, and lithium ions of nascent lithium carbonateparticles are easily coordinated with carbonate, sulfate and silicateradicals to form complex salts, so that outside the lithium ions, alayer of acid radicals forms, outside the layer, a layer of lithium ionsforms, followed by a layer of acid radicals, over and over again, andthey rapidly adhere to one another to form a cluster; these lithium ionswill also adhere to the inner wall of the glass-lined reactor or stirrer(as well as laboratory glassware, tools) made of silicates, which is arapid entropy-increasing process with great driving force. However, astime goes by, the cluster is loosened and broken due to continuousadjustment of various internal chemical bonds, the sulfate radicals inthe cluster are continuously bound to sodium ions and dissolved in hotwater, and lithium carbonate particles are continuously separated out,but it is hard to avoid such a case that a few or a very few clustersstill adhere to the inner wall of the reactor or stirrer, or areretained in the coarse lithium carbonates, resulting in a relativelyhigh content of sulfate radicals in a product and other problems.Therefore, this should be avoided to the utmost extent. b. If thedesiliconization of the lithium sulfate and sodium carbonate solutionsis not sufficient, lithium silicate is generated during the thermalprecipitation, which is very viscous and will increase the self-adhesionof lithium carbonate particles to make them easy to agglomerate afterbeing dried. This is because liquid lithium silicate has acharacteristic of never re-dissolving in water once dehydrated, and isvery different from sodium water glass, i.e. sodium silicate (forexample, liquid lithium silicate, as a concrete sealant, is very firmafter being dried and cured in construction, and is not afraid oflong-term soaking in water).

Therefore, the lithium sulfate leaching solution and the sodiumcarbonate solution both need to be strictly and effectivelydesiliconized, such that the solutions can be confirmed to be acompletely purified solution. The “supplementary impurity removal bypre-precipitation” rescue method is better to be on standby at any time,but it should be noted that during the spraying of a small amount ofprecipitant sodium carbonate in a mist form through pressurizedsprinkler feeding ports arranged reasonably at a proper speed under thecondition of vigorous and effective stirring, the reaction solutionsystem should be kept at pH<7.0; and the inner wall of the stainlesssteel reactor and the surface of the stirrer should be smooth and freeof scratches or spot welding slags, so as to prevent the lithiumcarbonate particles from being bonded.

Based on the principles of the adsorption and desorption, the thirdtechnique “high-efficiency desorption” of the invention is developed ina reasonable way, by which most of the sulfate radicals deeply coatedwith lithium carbonate particles in the initial stage of the thermalprecipitation can be successfully desorbed, so that the content of thesulfate radicals is further and substantially reduced to 0.03%-0.02% forthe industrial-grade lithium carbonate and 0.008% for the battery-gradelithium carbonate.

Here, the supplementary explanations are given for the technicalprinciple in paragraph [0040] that by properly increasing the feedingspeed of the completely purified lithium sulfate solution and moving thethermal aging duration backward to the thermal precipitation process,the coarse lithium carbonate with small particle size is obtained by the“thermal precipitation with small temperature increases and thermalstirring washing” procedure in the initial stage, and then the refinedlithium carbonate with large particle size and extremely low content ofsulfate radicals generated by recrystallization is obtained by thethermal aging in the “medium-high temperature strong desorption”procedure in the later stage: in a liquid-solid phase system, thegeneral technical principle of providing the conditions of “diluting,slowing, stirring, heating and aging” to obtain crystals with largeparticle size, so as to reduce the total surface area of the crystalsand reduce the adsorption of other harmful impurities on the surfaces ofthe crystals is also applicable to the coarse lithium carbonate obtainedby the thermal precipitation reaction of lithium sulfate solution withsodium carbonate solution. The “diluting”, although can reduce theconcentration of the sulfate radicals in a thermal precipitationreaction solution system, which is beneficial to reducing the absorptionof the sulfate radicals by lithium carbonate particles; however, it willreduce the first pass yield of the lithium carbonate, and the lowconcentration of sodium sulfate mother liquor is unfavorable forrecovering the byproduct sodium sulfate; the “slowing, stirring, heatingand aging” conditions has been adopted in the stage of reducing thecontent of sulfate radicals to 0.20% for the industrial-grade lithiumcarbonate and 0.08% for the battery-grade lithium carbonate, and provedto be effective and free of errors. However, in order to further reducethe content of sulfate radicals to a large extent to 0.03%-0.02% for theindustrial-grade lithium carbonate and 0.008% for the battery-gradelithium carbonate, or even to produce a 4N-grade product, the “slowing”operation needs to be modified to “moderately accelerating” operation,and the “aging” operation needs to be moved backward to the subsequentprocess to obtain crystals with small particle size first.

What does that mean? In the high concentration of thermal precipitationreaction solution system, impurities such as sulfate radicals and sodiumions in the peritectic crystal formed in the initial stage of thethermal precipitation are located at the core of the large crystal, andare extremely difficult to release by the thermal stirring washing, oreven the “strong desorption” (the thermal stirring washing is also adesorption operation substantially, but with much less strength than the“strong desorption”), and it is mainly these sulfate radicals thatcontribute to the content of 0.08% for the current battery-grade lithiumcarbonate standard. Therefore, the method of firstly obtaining crystalswith small size in the thermal precipitation procedure is performed tolay a technical foundation for the subsequent “medium-high temperaturestrong desorption” process, so that the technical problem can be solved.

Since the distance between the core and the outer surface of the crystalwith small size is small, and sulfate radicals and other impurities inthe peritectic crystal are buried shallowly, which belongs to “shallowcoating”, in the “medium-high temperature strong desorption” and thermalaging operation, most of the impurities is easily released in the liquidphase along with the process of the recrystallization of the crystalwith small particle size into a crystal with large particle size in anenvironment of very low concentration of impurities such as sulfateradicals caused by deionized water with a mass that is 5-6 times or morethe mass of the crystal with small particle size. Therefore, thistechnical foundation can improve the reliability of the “medium-hightemperature strong desorption” for further substantially reducing thecontent of sulfate radicals, moderately reduce the operation pressure,shorten the pressure operation, reduce the pressure reactor volume, andreduce the equipment investment.

The method is completely not concerned about the condition that thenumber and the total surface area of the lithium carbonate particleswith small particle size are substantially increased in the “thermalprecipitation with small temperature increases” procedure, andimpurities such as sodium carbonate and sulfate radicals are temporarilyand mostly adsorbed on the outer surface layer of the lithium carbonateparticles with small particle size, because the impurities are “buried”shallowly, most of which is easily separated from the coarse lithiumcarbonate-1 particles during the first thermal stirringwashing-centrifuging operation of the coarse lithium carbonate, and asmall quantity of residues are not sufficient to resist the strongdesorption capacity of the “medium-high temperature strong desorption”,and will be released together with the impurities such as the sulfateradicals which are deeply coated.

IV. DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a process flow of a spodumene-sulfuricacid process of the former Lithium of America corporation;

FIG. 2 shows a washing curve of sulfate radicals of a pilot productaccording to the spodumene-sulfuric acid process of the former Lithiumof America corporation;

FIG. 3 shows the solubility data for lithium phosphate, lithiumfluoride, lithium carbonate in water; and

FIG. 4 shows the decline curves of the content of sulfate radicals inlithium carbonates after the implementation of the invention.

The drawings 1-4 are detailed as follows: FIG. 1 is a drawing from AusTroche C., The Chemistry and Technology of Lithium [M]. Beijing: ChinaIndustrial Press, May 1965, first edition, page 160. Combining with thewritten part of this book, it can be clearly demonstrated that thethermal precipitation operation using the completely purified lithiumsulfate and sodium carbonate solutions is performed according to the“forward feeding” process.

FIG. 2 shows a washing curve of sulfate radical of a pilot productaccording to the “forward feeding” procedure of the thermalprecipitation in the spodumene-sulfuric acid process for producinglithium carbonate of the former Lithium of America corporation by theinventor of the present application in the initial stage of leading thesmall-scale industrial production by the spodumene-lithium sulfateprocess in 1978-79-80. The curve clearly shows that the content ofsulfate radicals is extremely difficult to further reduce after it isreduced to about 0.35% by washing, fully indicating that the biggestshortcoming of this conventional process is the high content of sulfateimpurity. The washing conditions are as follows: coarse lithiumcarbonate:distilled water=1:1.5, the temperature is 90-95° C., thestirring time is 30 minutes, and the SS-800 three-legged centrifuge isused for spin-drying at 1,000 rpm/min.

FIG. 3 shows the solubility data for lithium phosphate, lithiumfluoride, and lithium carbonate in water, which presents greatdifferences of an order of magnitude sequentially, indicating thatlithium recovery from lithium-containing sodium sulfate mother liquor ishighest in a lithium phosphate way.

FIG. 4 shows the decline curves of the content of sulfate radicals afterthe implementation of the “reverse feeding without mother liquorcirculation”, “supplementary impurity removal by pre-precipitation” and“high-efficiency desorption” techniques of the invention, both showing aprecipitous drop. In the figure, a character A represents the “thermalprecipitation with small temperature increases” stage, and a character Brepresents the “thermal stirring washing with small temperatureincreases”, the “medium-high temperature strong desorption” and“hydrocyclone separation” stages; the horizontal line represents thatthe coarse lithium carbonate-1 of the product in the stage A istransferred to the operation in the stage B to produce refined lithiumcarbonate; the left curve is for the industrial-grade lithium carbonateand the right curve is for the battery-grade lithium carbonate.

V. DETAILED DESCRIPTION

The invention will be further described with reference to specificembodiments. It should be understood that the following examples aremerely exemplary illustration and explanation of the invention, andshould not be construed as limiting the protection scope of theinvention. All techniques implemented based on the aforementionedcontent of the invention are included in the protection scope of theinvention.

Taking the direct production of lithium carbonate by thespodumene-sulfuric acid process as an example: when the technicalschemes of the invention are implemented, the current impurity removalmethod before the “thermal precipitation with small temperatureincreases and thermal stirring washing” procedure remains basicallyunchanged, and the “supplementary impurity removal by pre-precipitation”technique of the invention can be used for supplement; the “thermalprecipitation with small temperature increases and thermal stirringwashing” procedure must adopt the “reverse feeding without mother liquorcirculation” technique; the current techniques for drying, crushing,metering and packaging the refined wet lithium carbonate remainunchanged; all detection methods remain unchanged; the method fordetermining the end point of the “medium-high temperature strongdesorption” in paragraph [0046] only relates to sampling measures and amethod for calculating the content of sulfate radicals in solid-phaselithium carbonate, and does not relate to the change of the detectionmethod for sulfate radicals.

Here, the specific embodiments will be described in combination with the4 combinations of the three techniques of the invention described inparagraphs [0014-0015]:

{circle around (1)} The specific embodiment of producing the currentindustrial zero-grade lithium carbonate with 0.20% sulfate radicalsaccording to the combination 1:

The “supplementary impurity removal by pre-precipitation” in paragraph[0022] is performed as follows:

A purified lithium sulfate solution is added into a thermalprecipitation reactor, the stirring is started, and a small amount ofpurified sodium carbonate solution is sprayed in a mist form at a mediumspeed under the pressure of 0.05 MPa through pressurized sprinklerfeeding ports; the feeding is stopped once the reaction solution becometurbid followed by the precipitation of white fine substances (withyellow and red light when iron content substantially exceeds theacceptable level) as detected by naked eyes and a turbidity meter, andthe stirring is continued for a few minutes; the sample is taken andprecisely filtered, and then measured for the content of impurities suchas iron, aluminum, magnesium, calcium, and heavy metals, and if thecontent of impurities is unqualified, a small amount of sodium carbonateis sprayed, and the sample is detected again until the content ofimpurities is qualified, after which the stirring is continued foranother 15 min.

The qualified purified lithium sulfate solution is filtered, and theinitial filtrate is temporarily added into a small turbid solution tank(the volume of which is about 20% of the volume of the purified lithiumsulfate solution), and filtered again in a circular mode until theresulting filtrate is detected to be qualified, such that the filtercake is regarded as successfully bridged, and the filtrate is confirmedto be a completely purified solution.

Then, the specific operations of the “reverse feeding without motherliquor circulation” described in paragraphs [0018]-[0019] are performedto produce the current industrial zero-grade lithium carbonate with0.20% sulfate radicals. The equivalence ratio of the sodium carbonate tothe lithium sulfate should be 1.05.

For manufactures with advanced processes, equipment and instrument ofproduction and detection, and management, on the basis of their currenttechniques, the current industrial zero-grade lithium carbonate can besimply produced by the “reverse feeding without mother liquorcirculation” process with medium-low concentration of completelypurified lithium sulfate solution with 10% or slightly more sulfateradicals without combination with the “supplementary impurity removal bypre-precipitation”.

{circle around (2)} The specific embodiment of producing the industrial“new first-grade” lithium carbonate according to the combination 2:

The “new first-grade” lithium carbonate with 0.10% sulfate radicals canbe produced by applying the operations of the “reverse feeding withoutmother liquor circulation” process in paragraphs [0018]-[0019], the“thermal precipitation with small temperature increases and thermalstirring washing” operation of the “high-efficiency desorption” processin paragraphs [0041]-[0042], and optional operations of the“supplementary impurity removal by pre-precipitation” process inparagraph [0077] if necessary.

{circle around (3)} and {circle around (4)} The specific embodiment ofproducing the industrial “new zero-grade” and “new battery-grade”lithium carbonates according to the combinations 3 and 4:

According to the quality of the purified lithium sulfate solution, ifthe “supplementary impurity removal by pre-precipitation” is required,the purified lithium sulfate solution is purified supplementarily as perthe embodiment described in paragraph [0077].

As described in paragraphs [0041]-[0042], the completely purified sodiumcarbonate solution is added to a reactor for the thermal precipitationwith small temperature increases, and heated; a manhole on the reactoris covered, and the reactor is sealed after air in the reactor iscompletely driven out; when the reactor is heated to a selectedtemperature, such as 105° C. (0.13 MPa) for the industrial “newzero-grade” lithium sulfate and 118° C. (0.18 MPa) for the industrial“new battery-grade” lithium sulfate, the stirrer is started and is keptworking effectively, the completely purified lithium sulfate solution issprayed into the reaction in a mist form under a pressure of 0.1-0.3 MPaat a speed twice as fast as that of the original process for obtainingcoarse lithium carbonate particles with large particle size throughpressurized sprinklers arranged at multiple points, so as to firstlyobtain the coarse lithium carbonate-1 with small particle size. Once thefeeding is completed, the reactor is de-pressurized (a pipeline shouldbe connected to recover steam heat), after which the reaction solutionis immediately discharged, and centrifuged and rinsed to obtain thecoarse lithium carbonate-1, so that the “thermal precipitation withsmall temperature increases” is completed. The coarse lithiumcarbonate-1 is immediately transferred while hot into a reactor for thethermal stirring washing with small temperature increases into whichdeionized water at 90-95° C. with a mass that is 3 times (for theindustrial “new zero-grade” lithium carbonate) or 4 times (for the “newbattery” lithium carbonate) the mass of the coarse lithium carbonate-1has been added with the stirring started; a manhole on the reactor iscovered, and the reactor is sealed after air is completely driven out,heated to a selected temperature such as 105° C. (0.13 MPa) for theindustrial “new zero-grade” lithium carbonate and 120° C. (0.20 MPa) forthe “new battery-grade” lithium carbonate, and subjected to the thermalstirring washing for 15 minutes, then de-pressurized, and after thereactor is cooled to 95° C., the reaction solution is discharged,centrifuged and rinsed to obtain the firstly washed coarse lithiumcarbonate-2 with the content of sulfate radicals reduced to 0.30%-0.20%for the industrial “new zero-grade” lithium carbonate and 0.15%-0.10%for the “new battery-grade” lithium carbonate for later use, so that the“thermal stirring washing with small temperature increases” operation iscompleted.

Lithium salts such as coarse lithium carbonate or lithium phosphate andmirabilite or anhydrous sodium sulfate are recovered from the primarysodium sulfate hot mother liquor generated in the “thermal precipitationwith small temperature increases” process according to the operation ofthe “without mother liquor circulation” process.

Under the low-speed stirring, the coarse lithium carbonate-2 istransferred while hot into a reactor for the medium-high temperaturestrong desorption into which deionized water (for the battery-gradelithium carbonate, deionized water with the purity of 18 MΩ·cm,self-made and preheated to 90-95° C. is adopted) with a mass that is3-4-5 times (industrial-grade) or 5-6 times (battery-grade) that of thecoarse lithium carbonate-2 has been pumped with the low-speed stirringstarted; the reactor is heated, and completely sealed after air in thereactor is driven out; when the industrial-grade lithium carbonate is tobe produced, the reactor is heated to 144-159° C. (corresponding to thesaturated steam pressure of 0.4-0.6 MPa in the reactor), and when thebattery-grade lithium carbonate is to be produced, the reactor is heatedto 165-170-180° C. (corresponding to the saturated steam pressure of0.7-0.8-1.0 MPa in the reactor), and the “medium-high temperature strongdesorption” and the thermal aging operation are continued for 1 hour ormore with low-speed stirring, temperature and pressure maintained, so asto obtain lithium carbonate crystals with large particle size formed byrecrystallization; during the process, a small amount of liquid phase isextruded out through a specially arranged sampling pipe opening atregular intervals of time for rapid detection of the content of sulfateradicals (continuous detection on-line is performed as far as possible),and the content of residual sulfate radicals in the lithium carbonate inthe reactor is calculated accordingly; after the content is qualified, aheating valve is closed, the low-speed stirring is maintained, coolingwater is slowly and carefully introduced into a jacket to cool thereactor, when the pressure in the reactor is reduced to 0.05-0.06 MPa,the stirring speed is increased until the slurry is under a strongstirring condition, and the slurry is pumped into a hydrocycloneseparator with the speed controlled to continuously separate the liquidphase from the solid phase; the separated liquid phase contains slightlywater soluble impurities and particulate water insoluble impuritieswhich are adsorbed and coated in the coarse lithium carbonate, so thatit cannot be circularly used for the initial operation of the thermalprecipitation of the coarse lithium carbonate, and should be sent backto the leaching process, or sent back to the leaching process after thefilter cloth and equipment are cleaned; only part of the separatedliquid phase which is fully coagulated and subjected to precisefiltration is allowed to be mixed into deionized water for the thermalstirring washing of the coarse lithium carbonate-1 to produceindustrial-grade products, and prohibited in the subsequent processes.After the solid phase is subjected to the centrifuging-washing, therefined wet lithium carbonate is obtained. For the industrial “newfirst-grade” and “new zero-grade” lithium carbonates, the content ofsulfate radicals should be 0.10% and 0.03%-0.02%, respectively, and forthe “new battery-grade” lithium carbonate (subjected to the thermalstirring washing one more time if necessary), the content of sulfateradicals can be expected to reach 0.010%-0.008%-0.005%, or even canreach the limit of the main content of the 4N-grade lithium carbonateunder the optimal conditions.

The “high-efficiency desorption” of the invention can be naturallyextended to the following similar technical fields, in addition to beingused to substantially reducing the content of impurity sulfate radicalsin the lithium carbonate precipitated from the lithium sulfate solutionand the sodium (potassium) carbonate solution as exemplified:efficiently removing the impurities that are difficult to remove byconventional washing methods due to chemical adsorption and deep coatingin the core of the crystals (or particles) of the insoluble or slightlysoluble target product separated out by the precipitation reaction oftwo or more soluble inorganic substances, and therefore, these are allincluded in the technical scope of the invention.

1. A new method for substantially reducing the content of impuritysulfate radicals in lithium carbonate directly produced by lithiumsulfate and sodium carbonate (potassium), wherein, compared with theconventional method for reducing the content of sulfate radicals only bymultiple thermal stirring washings with deionized water, the method ischaracterized in that: taking a spodumene-sulfuric acid method fordirectly producing an industrial-grade lithium carbonate and abattery-grade lithium carbonate as an example, on the basis that most ofthe current techniques for removing impurities such as silicon, iron,aluminum, magnesium, calcium and heavy metals before the current thermalprecipitation process of manufacturers remain unchanged, the currenttechniques for drying, crushing, metering and packaging a refined wetlithium carbonate remain unchanged, and the current various detectionmethods remain unchanged, combinations of the following three techniquesof the invention are applied from a process of obtaining coarse lithiumcarbonate by a thermal precipitation reaction of a completely purifiedlithium sulfate solution with a completely purified sodium (potassium)carbonate solution to a process of obtaining the refined wet lithiumcarbonate: 1, an essential “reverse feeding without mother liquorcirculation” technique; 2, an optional “supplementary impurity removalby pre-precipitation” technique; and 3, an essential “high-efficiencydesorption” technique, so as to produce industrial zero-grade,industrial “new first-grade”, industrial “new zero-grade” and “newbattery-grade” lithium carbonates with corresponding contents of sulfateradicals reaching 0.20%, 0.10%, 0.03% and 0.008%, respectively.
 2. The“reverse feeding without mother liquor circulation” technique accordingto claim 1, wherein: the conventional operation of adding the completelypurified sodium carbonate solution into the completely purified lithiumsulfate solution in the thermal precipitation procedure is reversed,that is, the completely purified lithium sulfate solution is added intothe completely purified sodium carbonate solution, so that the chemicaladsorption and deep coating of sulfate radicals by lithium carbonateparticles are substantially reduced; a primary hot mother liquor ofcoarse lithium carbonate-1 obtained by centrifuging-rinsing is treatedin one of the following three ways, and not sent back to anacidification material leaching process: {circle around (1)} after theprimary hot mother liquor is cooled to 0° C. to −15° C. forcrystallization, and centrifuged to obtain mirabilite, another processroute is developed: a secondary cold mother liquor is concentrated untila sodium sulfate crystallization film is exactly formed, and filteredwhile hot to obtain the coarse lithium carbonate which is precipitatedagain, and sent back to the acidification material leaching process, anda tertiary hot mother liquor is combined with the primary hot motherliquor for the crystallization of mirabilite . . . , as such, theoperations of “cooling for precipitating mirabilite and heating forprecipitating coarse lithium carbonate” are alternately performed;{circle around (2)} the secondary cold mother liquor is concentrated invacuum to recover anhydrous sodium sulfate after the lithium isrecovered therefrom by precipitating water-insoluble lithium salts suchas lithium phosphate, lithium fluoride or lithium stearate; {circlearound (3)} the primary hot mother liquor is directly concentrated invacuum continuously to recover anhydrous sodium sulfate afterwater-insoluble lithium salts such as lithium phosphate, lithiumfluoride or lithium stearate are recovered therefrom, so that theconcentration of the sodium sulfate in a thermal precipitation reactionsolution system is substantially reduced, the beneficial technicaleffect of reducing the content of sulfate radicals by the “reversefeeding” is superposed, and the first pass yield of the coarse lithiumcarbonate-1 is improved due to the reduction of the salt effect.
 3. The“high-efficiency desorption” technique according to claim 1, {circlearound (1)} consisting of “strong desorption” and “hydrocycloneseparation” techniques, wherein the “strong desorption” further consistsof “thermal precipitation with small temperature increases and thermalstirring washing” and “medium-high temperature strong desorption”techniques; {circle around (2)} the “thermal precipitation with smalltemperature increases and thermal stirring washing” means that in caseswhere the current common jacketed reactor is still used for the thermalprecipitation and thermal stirring washing operation, under thecondition that the allowable pressure is 0.6 MPa for the jacket and 0.2MPa for the reactor, a preferred range of 0.13-0.18-0.20 MPa (but notlimited to a lower limit of 0.13 MPa) of saturated steam pressure in thereactor (corresponding to the temperature of 105-115-120° C. of thereaction solution) is selected for the thermal precipitation and thermalstirring washing (with 3 times, but not limited thereto, of deionizedwater added for the thermal stirring washing) to moderately reduce thecontent of sulfate radicals of the coarse lithium carbonate-1 and thecoarse lithium carbonate-2, moderately improve the first pass yield oflithium and moderately reduce the amount of deionized water used in thethermal stirring washing; {circle around (3)} the “medium-hightemperature strong desorption” refers to such a critical innovativetechnique that the coarse lithium carbonate-2 and deionized water with amass that is 3-4-5-6 times (but not limited thereto) that of the coarselithium carbonate-2 are added to a pressure reactor (taken as anexample), or a reactor or a reaction pipeline, and the mixture issubjected to the strong desorption and thermal aging for 1 hour or morein a preferred range of 0.5-0.6-0.7-0.8-1.2 MPa (but not limited tothereto) of saturated stream pressure (corresponding to the temperatureof 152-159-165-170-188° C.) under conditions of low-speed stirring,rolling, and constant low-speed movement of the solid phase of thereaction solution, so that part of sulfate radicals and most of theother water-soluble, slightly water-soluble and water-insolubleimpurities, which are deeply coated in the core of the coarse lithiumcarbonate due to the chemical desorption in the initial stage of thethermal precipitation reaction and thus are extremely difficult toremove, are desorbed and released in a large amount of deionized waterdue to intensified thermal motion of various molecules, ions and atomgroups in the solution system, and the lithium carbonate crystals withsmall particle size are recrystallized into large crystals withextremely low content of sulfate radicals in an environment with lowconcentration of sulfate radicals in the large amount of deionizedwater; {circle around (4)} and the “hydrocyclone separation” means thatmost of these water-soluble, slightly water-soluble and water-insolubleimpurities separated from the lithium carbonate particles by the“medium-high temperature strong desorption” operation are brought outdirectly with the rotating flowing liquid phase, instead of beingtrapped in the refined lithium carbonate particles, as is the case withvarious solid-liquid separation techniques using filter cloth or filterelements.
 4. The “thermal precipitation with small temperature increasesand thermal stirring washing” technique according to claim 3, whereinthe thermal precipitation operation is characterized in that: {circlearound (1)} the completely purified sodium carbonate solution is addedto a reactor for the thermal precipitation with small temperatureincreases and heated with the jacket opened; a manhole on the reactor iscovered, and the reactor is fully sealed after air in the reactor iscompletely driven out; when the reactor is heated to a selectedtemperature such as 105-115-120° C. (but not limited to the low limitrange of 105° C.), a stirrer is started and is kept working effectively,the completely purified lithium sulfate solution is rapidly sprayed in amist form into the reaction through pressurized sprinklers arranged atmultiples points (the feeding time is shortened by more than 50%compared with the conventional thermal precipitation process); thecoarse lithium carbonate-1 with small particle size is obtained, and thethermal aging duration of crystals with large particle size obtained bythe traditional process is moved backward to the “medium-hightemperature strong desorption” process in the initial stage, refinedlithium carbonate large crystals generated by recrystallization areobtained in an environment with a large amount of deionized water in thelater stage, so that most of impurities such as sulfate radicals deeplycoated in the core of the coarse lithium carbonate-1 particles due tothe chemical adsorption in the early stage of the thermal precipitationreaction are released; after the feeding is finished, the reactor isde-pressurized and cooled, when the temperature of the reaction solutionin the reactor is reduced to 90-95° C., the reaction solution isimmediately discharged, centrifuged and rinsed to obtain the lithiumcarbonate-2; {circle around (1)} the equivalence ratio of sodiumcarbonate to lithium sulfate is 1.05:1.00.
 5. The “thermal precipitationwith small temperature increases and thermal stirring washing” techniqueaccording to claim 3, wherein the thermal stirring washing operation ischaracterized in that: the coarse lithium carbonate-1 is transferredwhile hot into a reactor for the thermal stirring washing with smalltemperature increases into which deionized water with selected massmultiples such as 3-4-5 times for the industrial-grade lithium carbonateand 5-6 times for the battery-grade carbonate (but not limited thereto)has been added, is heated to 90-95° C. with a stirrer started; a manholeon the reactor is covered, and the heating is continued; the reactor issealed after air in the reactor is completely driven out, and subjectedto the thermal stirring washing for 15 minutes after being heated to aselected temperature such as 105-115-120° C. (but not limited to thelower limit of 105° C.), and then de-pressurized and cooled to 95° C.,after which the reaction solution is discharged, centrifuged and rinsedto obtain the industrial-grade coarse lithium carbonate-2 andbattery-grade coarse lithium carbonate-2 with the content of sulfateradicals controlled to be 0.30%-0.20% and 0.15%-0.10%, respectively, forlater use.
 6. The “medium-high temperature strong desorption” techniqueaccording to claim 3, wherein the operations are characterized in that:taking the use of a reactor of the medium-to-high-temperature strongdesorption as an example, deionized water with selected mass multiplessuch as 5-6 times (but not limited thereto) of the coarse lithiumcarbonate-2 (for producing lithium carbonate with purity more than99.950%, deionized water with the purity of more than 18 MΩ·cm isadopted) is pumped into the reactor, and heated with the jacket opened,with a low-speed stirring started, and the coarse lithium carbonate-2 isadded; the reactor is heated to a selected temperature such as 144-159°C. (0.4-0.6 MPa) for the industrial “new zero-grade” lithium carbonateand 165-170-180° C. (0.7-0.8 MPa) for the “new battery-grade” lithiumcarbonate (but not limited thereto), and subjected to the strongdesorption and thermal aging for 1 hour or more under the conditions ofthe low-speed stirring and the low-speed motion state of a slurry solidphase, so that water-soluble impurities mainly comprising sodiumsulfate, and other slightly water-soluble and water-insoluble impuritiesin the peritectic core of the lithium carbonate particles are releasedin deionized water, and a large number of lithium carbonate crystalswith small particle size are recrystallized into large lithium carbonatecrystals with the content of sulfate radicals of 0.03% for theindustrial “new zero-grade” lithium carbonate and 0.008% for the “newbattery-grade” lithium carbonate, respectively, in an environment withthe concentration of sulfate radicals far lower than the thermalprecipitation reaction concentration.
 7. The “hydrocyclone separation”technique according to claim 3, wherein: after the content of residualsulfate radicals in the lithium carbonate in the desorption reactor isdetected to be qualified in the “medium-high temperature strongdesorption”, a heating valve is closed, with the low-speed stirringmaintained, the reactor is de-pressurized and cooling water is slowlyintroduced for cooling, when the pressure in the reactor is reduced to0.05-0.06 MPa (but not limited thereto), the stirring speed is increaseduntil the slurry is under a strong stirring condition, and the slurry ispumped into a hydrocyclone separator with the speed controlled tocontinuously separate a liquid phase from a solid phase; the solid phaseis centrifuged and rinsed (for the “new battery-grade” lithiumcarbonate, the thermal stirring washing is performed one more time ifnecessary) to obtain the refined wet lithium carbonate (after drying),and for the industrial “new zero-grade” lithium carbonate and the “newbattery-grade” lithium carbonate, the content of sulfate radicals shouldbe reduced to 0.03% and 0.008%, respectively; the separated liquid phasecannot be circularly used for the initial operation of the thermalprecipitation of the coarse lithium carbonate, and should be sent backto the leaching process, or sent back to the leaching process after thefilter cloth and equipment are cleaned, and only part of the separatedliquid phase that has been subjected to the full coagulation ofimpurities and precise filtration is allowed to be mixed into deionizedwater for the thermal stirring washing of the coarse lithium carbonate-1to produce the industrial-grade products, and is prohibited in thesubsequent processes.
 8. The “medium-high temperature strong desorption”and the “hydrocyclone separation” techniques according to claim 3,wherein: in cases where a pipeline desorber is used to automatically andcontinuously perform the “medium-high temperature strong desorption”operation, the pressure of the slurry is reduced to 0.05-0.06 MPa (butnot limited thereto) through a de-pressurized storage tank with astirrer and a cooling water jacket, and the slurry is pumped into thehydrocyclone separator for the separation operation with the speedcontrolled.
 9. The “medium-high temperature strong desorption” techniqueaccording to claim 3, wherein the equipment is characterized in that:the main body of the desorption equipment can be selected from avertical pressure reactor with a stirring jacket, a low-rotating-speedspherical or horizontal cylindrical pressure reactor and a pipelinereactor, all adopting dividing-wall heating and cooling; the material ofthe part contacting the reaction solution (or whole) of the equipment,such as the inner wall or whole of a reactor, the outer wall or whole ofa stirrer, and the whole or inner wall of a pipe fitting, is selectedfrom 0Cr18Ni9Ti stainless steel, 0Cr18Mo2Ti stainless steel, glasslining, titanium material (reactor lining) and polytetrafluoroethylene(reactor lining); in cases where a reactor made of the 2 stainless steelmaterials is adopted to produce the battery-grade products, in additionto strictly controlling the content of fluorine in the purified lithiumsulfate solution and the content of chlorine in the purified sodium(potassium) carbonate solution obtained from fluorine-containing lithiumores such as fluor-lepidolite, there is also a strict restriction ofheavy metal impurities such as the magnetic metal chromium content ofless than or equal to 3 ppm, so the coarse lithium carbonate slurry mustbe firstly soaked in a small pressure reactor with pressure resistanceof 1.6 MPa at a saturated steam pressure of 0.8-1.0-1.2 MPa for a longtime (100 hours or more are recommended) to detect the leaching amountthereof, if the heavy metal content of the lithium carbonate exceeds anacceptable level after the soaking test, the batch of materials isrejected, and another selection is needed; in cases where a reactor withthe glass lining in the inner wall is adopted, a test with material isalso needed firstly to detect the dissolution amount of elements such asboron, aluminum, silicon, lead and antimony in the glass lining underconditions of alkaline lithium carbonate slurry, long time (100 hours ormore are recommended), high temperature (corresponding to a saturatedsteam pressure of 0.8-1.0-1.2 MPa) and low-speed stirring, if indexes ofthe impurities of the battery-grade lithium carbonate are unqualified,the formula comprising the glass lining in the inner wall is rejected,and another selection is needed; the material of part contacting thereaction solution of the “hydrocyclone separation” equipment selected isthe same as that of the desorber.
 10. The “high-efficiency desorption”technique according to claim 1, wherein: throughout the “thermalprecipitation with small temperature increases and thermal stirringwashing”, “medium-high temperature strong desorption” and ‘hydrocycloneseparation” operations, among all the listed technical parameters,except that the typical temperature range of 90-95° C. of theconventional thermal precipitation and thermal stirring washing belongsto the prior art, other characteristic parameters such as the amount andpurity of deionized water, control indexes of the saturated steampressure-temperature of the reactor, control indexes of the content ofsulfate radicals of the coarse lithium carbonate-1 and -2, rotatingspeed for stirring in the reactor, rotating speed of spherical andcylindrical horizontal desorbers, duration of the desorption and thermalaging, and hydrocyclone separation operation parameters are consideredas a whole instead of rigid parameters, and can be adjusted reasonablyand moderately within a certain range according to the grade of lithiumcarbonate to be produced, quality requirements of orders, componentcharacteristics of raw materials, yield and cost control, safetyproduction management and other factors; for example, for producing theindustrial-grade lithium carbonate, the amount of deionized water can bedistributed as appropriate according to a ratio of 2.5:5:0.5 or1.5+1.5:5.5:0.5 for the thermal stirring washing with small temperatureincreases, the “medium-high temperature strong desorption” and thecentrifuging-rinsing, and the total amount of the deionized water usedin the three processes with mass multiples of 8-9 times of the finishedlithium carbonate is sufficient; for producing the battery-grade lithiumcarbonate, the amount of deionized water can be distributed according toa ratio of 2.5:6:0.5 or 1.5+1.5:6.5:0.5, and the total amount ofdeionized water used in the three processes with mass multiples of 9-10times of the finished lithium carbonate is sufficient; for anotherexample, in the operation of the “thermal precipitation with smalltemperature increases and thermal stirring washing”, the control rangeof the saturated steam pressure-temperature parameter of a reactor is104.8° C.-0.13 MPa or 115.2° C.-0.18 MPa and up to 120.2° C.-0.20 MPa,which is relatively suitable for current thermal precipitation mainequipment of manufacturers and do not need to be replaced, in theoperation of the “medium-high temperature strong desorption”, thetemperature of 159-170° C. (0.6-0.8 MPa) is relatively suitable for tankand kettle main equipment, and in the automatic and continuous operationusing a pipeline desorber, the control range can be more than 170° C.(0.8 MPa) to 180° C. (1.00 MPa) and up to 188° C. (1.20 MPa), with noneed to exceed 200° C. (1.60 MPa) and entering the category ofmedium-pressure vessel management; therefore, the reasonably adjustedtechnical parameters are also included in the technical scope of theinvention intended to protect.
 11. The “supplementary impurity removalby pre-precipitation” technique according to claim 1, wherein: theoptional technique can be adopted in the following three situations: 1)if it is not found until the beginning of the thermal precipitationprocess that there is a mistake in the leaching operation or theoperation of removing impurities such as aluminum, iron, magnesium,calcium, and heavy metals by successive precipitation in the early stageor the equipment is faulty, such that colloid particles formed ofhydroxides of aluminum, iron, magnesium and certain heavy metals are notfully coagulated and thus incompletely precipitated, or the impuritiespass through the filter since the filter cloth is damaged and improperlyplaced, or other impurity removal accidents occur, and the indexes ofthe impurities of the purified lithium sulfate solution are detected tobe unqualified, then the technique can be adopted to perform efficientand convenient rescue; 2) the optional technique can be combined withthe “reverse feeding without mother liquor circulation” to produce thecurrent industrial zero-grade lithium carbonate; 3) the optionaltechnique can be used in the production of the industrial “newfirst-grade”, industrial “new zero-grade”, current battery-grade and“new battery-grade” lithium carbonates (including other high-puritylithium carbonates); particularly, if a process of circular leachingwithout lithium sulfate concentration is adopted, the colloid impuritiesof the hydroxides of aluminum, iron, magnesium and certain heavy metalsmay not be heated for a long time or the surface charges of colloidparticles may not be eliminated, and the impurities are not fullycoagulated and co-precipitated, and thus may pass through the filter, assuch, the technique can be adopted to perform efficient and convenientrescue.
 12. The “supplementary impurity removal by pre-precipitation”technique according to claim 11, wherein the operations arecharacterized in that: the purified lithium sulfate solution is addedinto a thermal precipitation reactor, the stirring is started, a smallamount of the completely purified sodium carbonate solution is sprayedin a mist form at a medium speed under the pressure of 0.05 MPa throughpressurized sprinkler feeding ports under the condition of closeobservation or on-line detection using a turbidity meter at 90-95° C.;the feeding is stopped once the reaction solution become turbid followedby the precipitation of white fine substances (with yellow and red lightwhen iron content substantially exceeds the acceptable level), and thestirring is continued for a few minutes; a sample is taken and preciselyfiltered, and then measured for the content of iron, aluminum,magnesium, calcium, heavy metals and silicon, if the content ofimpurities is unqualified, a small amount of completely purified sodiumcarbonate solution is sprayed into the reaction, and the sample ismeasured again until the content of impurities is qualified, after whichthe stirring is continued for another 15 minutes; the qualified purifiedlithium sulfate solution is filtered, and the initial filtrate istemporarily added into a small turbid solution tank (the total volume isabout 20% of the volume of the purified lithium sulfate solution), andcircularly filtered again until the filtrate sample is detected to bequalified, such that the filter cake is considered to be successfullybridged, and the filtrate is confirmed to be a completely purifiedsolution, which is then filtered again together with the purifiedlithium sulfate solution in the turbid solution tank until all purifiedsolution is converted into the completely purified lithium sulfatesolution; if the residue has a fine texture with a few coarse particles(lithium carbonate) as detected by naked eyes, the operation results arevery desirable.
 13. The combinations of the three techniques of theinvention according to claim 1, wherein: {circle around (1)} when thecurrent industrial zero-grade lithium carbonate with 0.20% sulfateradicals needs to be produced, the techniques for removing impuritiessuch as silicon, iron, aluminum, magnesium, calcium and heavy metalsbefore the current thermal precipitation process remain basicallyunchanged, and all detection methods remain unchanged, and only acombination of the “reverse feeding without mother liquor circulation”and the “supplementary impurity removal by pre-precipitation” issufficient; {circle around (2)} when the industrial “new first-grade”lithium carbonate with 0.10% sulfate radicals needs to be produced, acombination of the “reverse feeding without mother liquor circulation”,(or optional) “supplementary impurity removal by pre-precipitation” andthe “thermal precipitation with small temperature increases and thermalstirring washing” is sufficient; {circle around (3)} when the industrial“new zero-grade” lithium carbonate with 0.03% sulfate radicals needs tobe produced, on the basis of various impurity removal techniques for thecurrent industrial-grade lithium carbonate, only a combination of the“reverse feeding without mother liquor circulation” and the“high-efficiency desorption”, in combination with the “supplementaryimpurity removal by pre-precipitation” if necessary, is sufficient;{circle around (4)} when the “new battery-grade lithium carbonate” with0.01%-0.008% sulfate radicals needs to be produced, on the basis ofvarious impurity removal techniques for the current battery-gradelithium carbonate, only a combination of the “reverse feeding withoutmother liquor circulation” and the “high-efficiency desorption”, incombination with the “supplementary impurity removal bypre-precipitation” if necessary (the operation parameters selected suchas the amount of deionized water used, the temperature-pressureparameter, and the thermal aging duration are more stringent than thosein the production of the industrial “new zero-grade” lithium carbonate),is sufficient.
 14. The specification of the present application is onlyintended to illustrate and explain the contents of the invention bytaking the spodumene-sulfuric acid method as an example, which is stillthe largest scale production at present, and should not be construed aslimiting the application scope of the disclosure; in fact, the“high-efficiency desorption” technique of the invention can be naturallyextended to the following similar technical fields, in addition to beingused to substantially reduce the content of impurity sulfate radicals inlithium carbonate precipitated from lithium sulfate solution and sodium(potassium) carbonate solution, which is suitable to be exemplified:efficiently removing the impurities that are difficult to remove byconventional washing methods due to chemical adsorption and deep coatingin the core of the crystals (or particles) of the insoluble or slightlysoluble target product separated out by the precipitation reaction oftwo or more soluble inorganic substances, and therefore, these technicalfields are all included in the technical scope of the invention intendedto be protected.